create program modes using #if in c - c

Question

I am trying to create two modes for a program using #if and #define but the second mode isn't working why is that ?
I would aslo appreciate it if you could suggest a better way to do this.
Here's my code:
#include "Types.h"
#include <stdio.h>
void main (void)
{
u32 m;
u32 flag = 1;
do
{
printf("\nWelcome\nPress 1 for Admin mode\nPress 2 for User Mode\nYour Choice:");
scanf("%d",&m);
if (m==1)
{
#define m 1
flag = 0;
}
else if (m==2)
{
#define n 2
flag = 0;
}
else
{
printf("\nInvalid number \nPlease Try again");
}
}while(flag);
//using conditional directive to run only the portion of the code for the slected mode
#if m==1
printf("Welcome to admin mode");
#elif n==2
printf("Welcome to user mode");
#endif
}

Answer 1

The #define and ifs are part of the preprocessor macro.
One way to think about them is to imagine the compiler going through your files and cutting and pasting in them as an early step in compilation. When you define for example PI to be 3, it will paste 3's everywhere in your code that you have written the PI. This then tells us that it won't matter which branch of m == 1 or 2 we go down when running the program - all the preprocessor editing has already been completed!
One way to get the program built in a certain mode would be to use a flag when compiling, for example -D DEBUG. Note that we cannot use this to select mode in an already compiled program.
Preprocessor Options:
-D=
Adds an implicit #define into the predefines buffer which is read before the source file is
preprocessed.

Answer 2

The preprocessor if #if can only interpret values which are known at proprocessing time, which is even before compile time.
It cannot read values from variables like your u32 m.
On the other hand, a preprocessor #define is also only done at preprocessing time, it will not be influenced by being within the "then" branch or the "else" branch of an if.
Doing #defines within code blocks (e.g. if branches), or even within a function, is there for not recommended.
You did not specify how your code misbehaves, but I would not be surprised, if the #if always acts admin mode. There has been a #define m 1 before in the file (no matter what path runtime execution took), so the proprocessor will take the first option.

Answer 3

In the C language, al the directives starting with "#" are used by the preprocessor. The preprocessor scans your file before compilation so that "variable" m is hardcoded and you can't change it in runtime (when you're running the program).
Also, the "m" variable is declared but not used.
To change the behaviour of the program on runtime, you should use a standard variable and use a switch-case to check for the variable's value and run the appropriate code.
I would also recommend using standard types defined by the language like "int" or "char" as they have better portability through the different architectures.
Your code could be like this
#include <stdio.h>
int main (void)
{
int m;
do
{
printf("\nWelcome\nPress 1 for Admin mode\nPress 2 for User Mode\nYour Choice:");
scanf("%d",&m);
if (m == 1)
{
printf("Welcome to admin mode");
return 0;
}
else if (m == 2)
{
printf("Welcome to user mode");
return 0;
}
else
{
printf("\nInvalid number \nPlease Try again");
}
}while(m != 1 || m != 2);
return 0;
}